One example of a proposed bulb socket structure for use in an in-vehicle room lighting device provided at a ceiling of a vehicle such as an automobile, is provided with a socket body made of resin for mounting a bulb such as a wedge base bulb, and a pair of power-supply terminals (i.e. terminal clamps) provided at the socket body, wherein a base portion (i.e. a basal portion) of the bulb is inserted into a mounting hole of the socket body and is sandwiched by the pair of power-supply terminals (refer to Patent Literature 1, for example). For a bulb socket structure disclosed in Patent Literature 1, as shown in FIG. 8, a pair of power-supply terminals 101 is provided with a pair of holding pieces 102 for holding a base portion 111 of a bulb 110, so that by inserting the base portion 111 between the pair of holding pieces 102, the pair of holding pieces 102 is elastically deformed in a direction away from each other and holds the base portion 111 with a pushing force associated with this deformation. Furthermore, as shown in FIG. 9, there is provided a spacer 120 attached to a mounting hole 103 of a socket 100 and having a regulation wall 121. By allowing the regulation walls 121 to abut on the holding pieces 102 which are elastically deformed by the insertion of the base portion 111, the excessive deformation of the holding pieces 102 can be inhibited.